Abstract
The Seebeck coefficient, S, and electrical resistance, R, have been measured as functions of temperature at constant oxygen composition, z, for ceramic YBa 2Cu 3O z samples with 6.197 ≤ z ≤ 6.400 and at temperatures up to ≈ 1000 K, by means of a simple experimental device which allows us to change the temperature without changing the composition z. S( T) z exhibits a large peak at T ≈ 200 K which decreases as z increases. The main object of the discussion is to understand the origin of this peak. At first, it is shown that this S peak is independent of the Néel transformation which occurs in the above composition range. The coupling between the Néel process and the transport properties is revealed, as for similar systems, by a λ point on the ( ϑ ln R/ ϑT) z versus T plot at T N and by a positive hump for ln R( T) z . Several possibilities are then discussed, notably antimagnon and phonon drag. A new experimental fact appears as a key argument in this discussion. The system exhibits a Fermi glass behavior with an Anderson transition occurring on changing z. At the transition which is observed between z = 6.350 and z = 6.400, the activated diffusion component of S disappears. However, for z = 6.400, on the metallic side of the transition, a T −1 term is still observed in S which is the signature of phonon drag. This phonon-drag effect appears to be unusually strong: the maximum temperature of the S peak, up to 250 K, is high as regards the Debye temperature, ≈ 350–400 K, and the phonon-drag contribution to S amounts to about 30% at 600 K for z = 6.400. This indicates a particularly strong electron-phonon coupling. As a consequence, the coupling parameter λ is expected to be high in these systems.
Published Version
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